Reciprocating engines using a condensable vapor, usually steam, and with or without condensers, have been known and widely used for about two hundred years. For most of this period, a low inherent thermal efficiency was the price paid for relatively mild steam conditions, that is, low temperature and low pressure.
These mild steam conditions were for a long period dictated by the boiler for the condensable vapor. The fire tube boiler was simple, sturdy, and easy to operate and it is still in wide use. Even today, however, a fire tube boiler is limited to maximum pressures of about 250 psig. and much lower pressures are often used. The fire tube boiler can be used with a superheater, but the majority of reciprocating steam engines in use, until the virtual eclipse of the genre in the twentieth century, made use of saturated steam at pressures below 250 psig. These steam conditions allowed the use of simple inlet valves, reasonably effective under the conditions used, having a variety of designs such as slide valves, piston valves, and poppet valves, and a simple lubrication system.
A further feature of this prior art type of steam engine which also bought simplicity at the expense of efficiency, was a relatively small expansion ratio of steam and, in many cases, none at all. This simplified valve design and allowed easy inlet valve intervals.
The net result was an engine which was simple, sturdy, long lived, and required no exotic or unusual construction materials or techniques; however, the price paid was low efficiency.
In recent years, a considerable effort has been made to develop condensing steam reciprocating engines with much higher efficiencies. A natural approach, with predictable theoretical results, but still within the confines of the Rankine condensing cycle, has been to use much higher temperatures, pressures, and expansion ratios. Steam conditions at inlet of 1000.degree. F with pressures from 1000 to 3000 psia, and pressure ratios in expansion of 25 to 1, have been employed. New techniques and improved materials have been used and great progress has been made in rapid and efficient steam generation through the use of improved monotube type boiler-superheaters.
Another approach to obtain higher efficiency has been to alter the basic Rankine cycle. U.S. Pat. Nos. 3,798,908; 3,716,990 and 3,772,883 teach a condensing vapor cycle in which maximum operating pressure is attained by mechanical compression of wet vapor, i.e. two phase compression. This cycle shows significantly higher ideal efficiency than the Rankine cycle with identical vapor conditions at inlet and exhaust. This improved cycle has relatively high temperature as a basic requirement in order to show worth-while improvement over the Rankine cycle.
All these improved engine types, requiring high inlet temperatures and pressures, and very short inlet valve intervals, make heavy demands on both mechanical features and metallurgy. As expected, they show predictably higher efficiency than condensing engines operating with saturated steam at lower pressures. Very recent developments in steam engines for automotive use now show that even these improved efficiencies may be insufficient for modern vehicular use. Further projections based on still higher temperatures, pressures, and expansion ratios are now under consideration. Inlet temperatures of 1500.degree. F and pressures of 3000 psig are predicted with overall pressure ratios of 80 in the expansion process, requiring a compound engine with reheat. These conditions will require new frontiers in inlet valve material and mechanical design.
The net result is that the provision of suitable inlet valving sets one constraint on the reciprocating condensing vapor engine based on either the Rankine or the steam compression cycles. Another constraint is set by the requirement of upper cylinder lubrication. Rankine engines operating at steam inlet temperatures of 1000.degree. F have been shown to be capable of prolonged operation with monotube boilers using hydrocarbon-based oils for upper cylinder lubrication but it is extremely unlikely that this method will suffice at 1500.degree. F, much less at even higher temperatures.
A third constraint is economic--the high cost in strategic materials such as nickel and chromium required in monotube boiler-superheaters and reheaters operating at such elevated temperatures and pressures.
A way to obviate these problems is through the use of a condensing vapor engine using a modification of the Stirling cycle. This method is disclosed and claimed in U.S. Pat. application Ser. No. 596,165 filed July 15, 1975 now U.S. Pat. No. 3,996,745. This cycle makes use of the cooling effect of two-phase vapor compression as taught in U.S. Pat. Nos. 3,798,908; 3,772,889 and 3,772,883. Lubrication and piston sealing in the engine are similar to methods developed for high pressure Stirling engines using gaseous working fluids such as hydrogen and helium. In engines of this type, the piston is sealed by plastic rings at the bottom of a long cylinder, so designed that the ring always operates in a relatively cool portion of the cylinder, while the hot space of the cylinder and the top of the piston can be at very high temperatures in excess of 1500.degree. F. Such engines, of the so-called Rinia type, with interconnected hot and cold spaces, have no inlet valves at all; require no lubricants and in both the gaseous and condensing vapor type are mechanically simple as regards valve requirements.
The gaseous Stirling engine has neither inlet nor outlet valves in the normal mode of operation; while the condensing vapor type has an outlet valve for passing part of the condensable vapor to the condenser, and an injector for injecting condensate into the so-called cool space during compression. These are easy operations both as regards mechanical features and metallurgical requirements.
A negative feature of the Stirling cycle is the need to cycle the working substance in the gaseous state between hot and cold spaces in the engine. The combined effect of gaseous viscosity and inertia is to reduce the efficiency of the cycle when it is operating at maximum power, i.e. at maximum pressure, as the usual way to alter power output in such engines is to alter the pressure of the working substance.
A further practical problem in the Stirling engine, whether based on gaseous or condensable vapor working substance is design and fabrication of the heater elements between the hot and cold spaces of the engine. To date, no satisfactory compromise has been effected between material cost, engine efficiency, and the requirements of mass production.
Present practice is to use a tube bundle. The material of construction is generally high temperature alloy steel. Metallurgical requirements place a constraint on temperature, and the shape and configuration of the tubes places a further constraint on mass production methods.
Ceramics and cermets, however satisfactory for continuous high temperature operation in an oxidizing flame, pose difficult problems of fabrication.